Metal oxide semiconductor (MOS) power transistor devices have gained popularity in certain applications for their high power capabilities. There are various types of MOS power transistor devices, including diffused metal oxide semiconductor (DMOS) devices, such as lateral DMOS (LDMOS) devices, vertical DMOS devices and trench DMOS devices.
A MOS power transistor device may comprise one or more transistor dies mounted on a base. The die(s), base and potentially other components of the transistor device may be contained, at least partially, in a package. For example, the base is typically surrounded by polymer or ceramic walls that form cavity sidewalls of the package. A polymer or ceramic lid is then placed on top of the sidewalls. These “packaged” components are referred to herein collectively as a device package. The base, which typically comprises the bottom of the device package, may be mounted on a heat sink to remove heat from the device package during operation.
A method commonly used to mount the die on the base is soldering. During soldering, however, elevated temperatures are experienced causing both the die and the base to expand. The die and the base typically comprise different materials, and therefore expand by different magnitudes. For example, dies typically comprise silicon which, when heated, will expand by a lesser magnitude than the base, which typically comprises copper, or some alloy thereof. This differential expansion and its associated contraction, e.g., during cooling, can cause warping of the base.
Warping of the base is undesirable. For example, depending on the amount of warping that occurs, the device package may not be suitable for attachment to the heat sink. Specifically, attaching a warped base to a heat sink can result in voids, or gaps, being present between the base and the heat sink. These voids can serve as an impediment to the thermal path between the device package and the heat sink, and thus can lead to overheating of the device.
Therefore, techniques for minimizing or eliminating warping in power transistor devices, and other types of integrated circuit devices, would be desirable.